It has been widely recognized that multiple peak resonant tunneling diodes (RTDs) are useful in several circuit applications. These include multi-state memory, high speed analog-to-digital converters, multi-valued logic circuits, parity bit generators, and neural networks. For these applications, the multiple peak RTD device exhibits multiple equally-spaced peaks and valleys in its current-voltage (I-V) characteristics. A further requirement demands that the largest valley current not exceed the smallest peak current.
Conventional methods for fabricating a multiple peak RTD involve integrating serially connected multiple RTDs in an epitaxial stack. However, the number of peaks is limited by the accumulated series resistance of the undoped double-barrier structure, which produces undesirable hysteresis in its I-V characteristics. In addition, the RTDs' peak-to-peak and valley-to-valley separation cannot be made smaller than that of a single RTD. Another known multiple peak device is fabricated with multiple potential barriers and quantum wells, but this type of device is difficult to design to accomplish low valley currents in its I-V characteristics.
Accordingly, it has become desirable to provide a multiple peak RTD devices which possess desirable I-V characteristics at normal operating temperatures. Furthermore, in the interest of large scale integration, device compactness and ease in fabrication are additional important and desirable properties.